QM-MM Ehrenfest dynamics from first principles: photodissociation of diazirine in aqueous solution

FRANCISCO RAMIREZ; GONZALO DIAZ MIRON; MARIANO GONZALEZ LEBRERO; DAMIAN A SCHERLIS

THEORETICAL CHEMISTRY ACCOUNTS

SPRINGER

Lugar: Berlin; Año: 2018 vol. 137

1432-881X

This article describes an implementation of Ehrenfest molecular dynamics based on TDDFT and Gaussian basis sets,optimized for hybrid QM?MM simulations in GPU. The present method makes use of the equations of motion proposedby Chen et al. (J Chem Phys 135:044126, 2011), which, at variance with previous formulations of the Ehrenfest dynamics,takes into account the movement of the localized basis functions, thus improving accuracy and energy conservation. Thismethodology is used to explore the evolution and the stability of excited state dynamics for two different constructions ofthe initial excited state, consisting in the linear response TDDFT S1 solution, and in the ground state density matrix wherethe HOMO?LUMO occupancies have been switched, which is a widespread approach to model photoexcitation in electrondynamics simulations. It is found that the second kind of starting state leads to a larger numerical noise and to a poorerstability of the dynamics, aside from ?awakening? inner electronic modes that become manifest in the frequency spectrum,and which are absent if the dynamics departs from the linear response TDDFT density matrix. Then, the method is appliedto investigate the photodissociation of the diazirine molecule, CH2N2 , both in vacuum and in aqueous solution. Diazirinedecomposes into carbene and molecular nitrogen upon irradiation with UV light, and for this reason it has been widelyadopted to photolabel biomolecules through the insertion of carbenes in the macromolecular surface. Our simulations suggestthat the quantum yield of the dissociative reaction experiences a decrease in solution with respect to the gas phase, thatcan be understood in terms of the vibrational relaxation facilitated by the solvent molecules. Besides, the present resultsindicate that the isomerization and dissociation mechanism occur fully on the S1 excited state.